In most applications of lubricating oils which are to be used at elevated temperatures, it is desirable that the lubricating oil formulation exhibit good oxidation resistance, in order to minimize or prevent the increase in viscosity, formation of sludge and acidity of the lubricant, and the consequent lowering of the lubricating ability of the oil and lubricating system in general.
In the prior art, many materials have been disclosed to improve high-temperature oxygen stability and resistance to discoloration, including calcium naphtha sulphonates, barium versatates, calcium phenates, and various phenols, phosphates and phosphites. However, conventional stabilizing/antioxidant systems, which are typically used in naphthenic and solvent-refined lubricating oils, have shown limited success when used with certain primarily paraffinic lubricating oils, namely hydrotreated oils, poly-.alpha.-olefin oils, paraffinic white oils and mixtures thereof. Different lubricating oils do react in different ways to different antioxidant systems. As has been shown in the past, the effect of an antioxidant mixture in a lubricating oil is a function of the sulphur level and the aromatic content of the lubricating oil. As is discussed in more detail below, the hydrotreated oils used in this invention typically contain less than about 1 per cent total aromatics and, preferably, less than about 0.25 per cent aromatics. Further, these hydrotreated oils typically have a sulphur level less than about 50 ppm and, in some cases the oils may have a sulphur level less than about 1-2 ppm. On the other hand, sulphur-refined and naphthenic oils have corresponding levels which are at least one to two orders of magnitude larger. Hence there is a need for a reliable stabilizing system for use with hydrotreated oils, poly-.alpha.-olefin oils, paraffinic white oils and mixtures thereof.
Phosphites are known in the art as stabilizers for lubricating oils. In U.S. Pat. No. 3,652,411, Commichau discloses a mixture of phosphite, phenol, substituted amine, organic phosphate, polyhydroxyquinone and benzotriazole as a stabilizer for polyglycol lubricant. There was no discussion of subcombinations of this rather complex mixture. Orloff et.al. in U.S. Pat. No. 3,115,463 discloses the stabilization of mineral oils and synthetic diester oils by a synergistic mixture of dialkyl hydrogen phosphite and substituted phenol or bisphenol. U.S. Pat. No. 3,115,464 by the same inventors discloses an orthoalkyl phenol in admixture with dialkyl hydrogen phosphite, where the alkyl groups were isopropyl or tertiary butyl. Spivack et.al. in U.S. Pat. No. 4,374,219 discloses a phosphite stabilizer which was an alkanolamine ester of a non-cyclic and a cyclic phosphite. It was said to be useful as a stabilizer for lubricating oils and polymers, alone or in combination with selected hindered phenols. In U.S. Pat. No. 3,556,999, Messina discloses a stabilized hydraulic fluid containing a lubricating oil, a phosphite or disubstituted phosphate, a substituted phenol or an aromatic secondary amine and an oil-soluble dispersant copolymer. See also U.S. Pat. No. 3,115,465 by Orloff et.al. which discloses a further particular combination of phenols and phosphites. In particular, these latter two patents use phosphites which have a relatively low molecular weight and do not have low volatility as defined herein.
However, severely hydrotreated oils present particular problems for stabilizers in hot oxygen or air exposure of lubricating oils, as acknowledged in U. S. Pat. No. 4,385,984 of Bijwaard et.al. That patent discloses a hydrotreated oil having poor oxidation stability to which was added a substantial quantity of less severely hydrotreated oil containing some remaining sulphur. Nevertheless, there remains a need for a really effective stabilizer for use with hydrotreated oils, poly-.alpha.-olefins and paraffinic white oils.
In U.S. Pat. No. No. 4,025,486, Gilles discloses a stabilizer for stabilizing polyolefin polymers from degradation when exposed to ultraviolet light. The stabilizer comprises a mixture of hydroxyphenyl-alkyleneyl isocyanurates and a pentaerythritol phosphite. As is apparent from the foregoing, the isocyanurates were used as a stabilizer to prevent degradation from ultraviolet light. Further, the stabilizer was for use with a polymer and not a lubricating oil.
In U.S. Pat. No. No. 4,652,385 and corresponding Canadian Pat. No. No. 1,248,516, there is disclosed a lubricating composition comprising a major amount of lubricating oil selected from the group consisting of hydrotreated oil, poly-.alpha.-olefin oil and paraffinic white oil, and an antioxidant amount of a synergistic mixture of:
(a) a low-volatility, hydrolytically stable, organically substituted phosphite or diphosphite, wherein the substituent groups are alkyl, aryl or alkylaryl, and said phosphite contains substantially no hydroxy groups, and PA0 (b) a low-volatility sterically hindered phenolic compound. PA0 (a) a low-volatility, hydrolytically stable, organically substituted phosphite or diphosphite, wherein the substituent groups are alkyl, aryl or alkylaryl, and said phosphite contains substantially no hydroxy groups, and PA0 (b) tris (2-hydroxyethyl-3,5-di-tert-butyl-hydroxy-cinnamate) isocyanurate or tris (3,5-di-tert-butyl)-4-hydroxybenzyl isocyanurate.
In particular, these patents disclose a synergistic mixture wherein the phenolic compound is selected from the group having the formulae: ##STR1## wherein R1 and R2 are, independently, isopropyl or tertiary butyl, and n is 2, 3, or 4, and ##STR2## where R1 and R2 are, independently, isopropyl or tertiary butyl.